The cardiac sodium channel Na_V1.5 initiates the action potential (AP) and is essential for conduction. The last three residues of Na_V1.5 (Ser-Ile-Val) constitute a PDZ domain-binding motif (SIV) that interacts with PDZ proteins such as syntrophin proteins and SAP97 defining distinct subsets of Na_V1.5 multi-protein complexes, respectively at the lateral membrane and the intercalated discs (ID) of cardiomyocytes. Previous results with dystrophin-deficient mice showed that disruption of the dystrophin-syntrophin macromolecular complex impairs excitability and impulse propagation by reducing the expression of Na_V1.5 and syntrophin at the lateral membrane of cardiomyocytes. Recently, we investigated the in vivo role of the Na_V1.5 SIV motif by characterizing mice bearing a truncation of this motif (ΔSIV). Western blots of ΔSIV hearts displayed reduced levels of Na_V1.5, which corresponded to a 35% decrease in cardiomyocyte wholecell I_Na and AP upstroke velocity. Immunostainings revealed a specific loss of Na_V1.5 channels in ΔSIV cardiomyocytes at lateral membranes concomitant to a 60% decrease in I_Na recorded at the lateral membrane. However, Na_V1.5 expression at ID and T-tubules was not altered. No I_Na decrease was observed at the ID. Epicardial mapping of ΔSIV hearts showed decreased conduction velocity that manifested as prolongation of the QRS interval in ECGs. When SAP97 expression was constitutively suppressed in the heart of knock-out mice, I_Na and AP maximal upstroke velocity were unchanged. However, AP duration was greatly increased, suggesting that the expression and function of other ion channels is modified. These data reflect the in vivo significance of the PDZ-domain-binding SIV motif in correct expression of Na_V1.5 channels at the lateral membrane and underline the functional role of lateral Na_V1.5 channels in cardiac conduction. In addition, the results confirm the existence of distinct pools of Na_V1.5 channels within the cardiomyocytes and suggest a complex regulation and organization of the Na_V1.5 pool at the ID, where compensatory mechanisms could be developped when SAP97 expression is constitutively suppressed.